The present invention relates to an optical head device for irradiating a light to an information recording carrier or medium to optically record or reproduce information.
A compact disk which records a digitized audio signal by roughness of a surface and a video disk which records a TV video signal have been rapidly populated.
In a compact disk player, not only a stationary type but also portable type or vehicle mount type are expected.
User's requirements to such optical information recording and reproducing apparatus are thin, compact and inexpensive. Same requirements are imposed to the optical head device which is mounted in such an information recording and reproducing apparatus and which records or reproduces information on or from the information carrier.
FIG. 1 shows a prior art optical head device.
In FIG. 1, a dispersing light flux emitted from a laser light source 1 is directed to a collimator lens 2 where it is collimated and directed to a polarization beam splitter 3, which transmits substantially 100% of linear polarized light having a vibration plane in a specific direction and reflects substantially 100% of linear polarization light having a vibration plane orthogonal to said plane.
The linear polarization light transmitted through the polarization beam splitter 3 passes through a .lambda./4 plate and is converted to a circular polarization light, which is focused by an objective lens 5 onto an information recording plane 7 formed on a substrate 6 of the information carrier to form a spot of around 1 .mu.m in diameter.
The light flux reflected by the information recording plane 7 passes through the objective lens 5 by which it is collimated, and it passes through the .lambda./4 plate 4 by which it is converted to a linear polarization light having a vibration plane orthogonal to that for an incident light thereto. The light is then directed to the polarization beam splitter 3. The polarization beam splitter 3 acts as a light splitter because of the property described above and reflects the light reflected from the information recording plane 7 to separate it from the incident light, and directs the reflected light to a photo-detector 10 as a focused light flux through a sensor lens 8 and a cylindrical lens 9.
When the information is to be recorded by using the optical head device, the laser light source 1 is driven in accordance with the information signal to modulate an intensity of an incident light to the information recording plane 7 to record the information. When the information is to be detected, a non-modulated light is irradiated to the information recording plane 7 having information recorded thereon by pits or changes of reflection coefficient and the reflected light modulated by the recorded information is detected by the photo-detector 10 so that the information is reproduced. The photo-detector 10 has a photo-sensing plane thereof divided into four sections, and a focus error signal is detected by a combination with a cylindrical lens 9 by a known method (so-called astigmatism method).
However, in the prior art optical head device, it is necessary to arrange many optical elements such as polarization beam splitter and photo-detector in a space in a predetermined relationship, and mutual positioning is complex.
Further, because of a large number of parts, it is difficult to reduce the size of the device and many manufacturing steps are required and hence the device is expensive.
It is an object of the present invention to provide an optical head device which resolves the problems encountered in the prior art device, is suitable for size reduction and weight reduction, and can be manufactured at a low cost.
The above object of the present invention is accomplished by the optical head device comprising a light source, a light splitter for reflecting a light emitted from the light source by a first plane to direct it to an information carrier and transmitting a light from the information carrier through the first plane, a photo-detector for detecting the light from the information carrier, and a diffraction grating structure for diffracting the light transmitted through the first plane to a second plane of the light splitter other than the first plane to direct the light to the photo-detector.